Access control devices of the above-cited kind are preferably used in large service or industrial installations having different departments that should be accessible to only a defined group of persons. In such devices, each authorized person is assigned an individual identification number which is broadcast continuously when the portable transmitter is switched on. A locally fixed receiver has a memory containing the identification numbers of the authorized persons and also has a comparator circuit that generates an output signal when a received ID number corresponds to a stored ID number. This output signal causes the opening of the locking mechanism to admit one person.
Such devices require a relatively complicated portable transmitter whose carrier frequency is modulated with the individual ID number and they do not operate with the desired reliability under all external conditions because the voltage of the battery that supplies the current for the portable transmitter is highly temperature-dependent. Hence, these devices are not suitable for controlling access to installations where the authorized persons change daily or even several times on the same day nor to installations whose entrances are subjected to low temperatures, especially in the winter.
Accordingly, it is the object of the present invention to create an access control device with a portable HF transmitter that is of simple construction, is powered by a temperature-independent current source and can be made no cheaply, even including its auxiliary parts, as to be suitable for only a single use.
This object is attained, according to the invention, by using an access control device of the type described at the outset but characterized in that the local control device includes a local computer in which all valid control codes are stored as well as an HF transmitter whose carrier wave is modulated with the continuous sequence of valid ID numbers and further characterized in that the portable control pass includes an electronic memory storing the ID number assigned to the control card, as well as a comparator circuit and an HF transmitter which sends out a recognition signal when the comparator determines correspondence between the assigned ID number and an ID number from the sequence of ID numbers transmitted by the HF transmitter of the control device, the recognition signal being fed from the HF transmitter of the control device to the local computer which controls the locking mechanism.
In a preferred embodiment of the control device, the portable control pass contains a current supply circuit which converts the energy received from the HF transmitter of the local control device into the DC current required for the operation of the circuit elements of the control card.
The access control device according to the invention is especially suitable for use in recreation sites and, for example, in regional skiing centers that include several closable site entrances or partial site entrances, each of which cooperates with an associated local computer.
The control pass used with the access control device according to the invention can be made cheaply because it contains only simple electronic circuits and components and sends out an un-modulated signal, all complicated and expensive circuits being associated with the locally fixed transmitter. Moreover, a valid pass permits unhindered passage through the locking mechanism because the locally fixed transmitter and receiver cooperate with the receiver and transmitter of the control pass without any action by the control pass bearer.